1. Field of the Invention
The present invention relates to a heating control device, a fixing device, an image forming apparatus, a heating control method, and a computer program product.
2. Description of the Related Art
A fixing device has been known in which a pair of rotating members facing to each other are heated by respective heat sources, one of the rotating members is pressed at a predetermined pressure to form a nip between both the rotating members, and paper is allowed to pass through the nip, and an image is fixed on the paper.
For example, Japanese Patent Application Laid-Open No. 2002-221871 discloses an image forming apparatus in which a heater for a heating roller is controlled by detection outputs from a temperature sensor of the heating roller or a temperature sensor of a pressure roller. More specifically, the heater of the heating roller is controlled based on the temperature of the pressure roller and the heater of the heating roller is controlled based on the temperature of the heating roller. In addition, the heater of the heating roller is controlled based on the temperature change of the pressure roller.
As another example, Japanese Patent Application Laid-Open No 2003-149987 discloses a technology in which a target temperature of the heating roller is calculated based on the detected temperature of the pressure roller, and the heat source of the heating roller is controlled based on the calculated target temperature of the heating roller and the detected temperature of the heating roller.
In both of the conventional technologies, each of the heat sources for the heating rollers is controlled based on the detected temperature of the pressure roller. However, turn-ON and OFF of the heat source of the heating roller is merely performed (or input is reduced) based on the detected temperature of the pressure roller.
In some fixing devices, fixing control has been performed with the use of proportional-integral-derivative (PID) control; however, the control is performed using a temperature sensed by a single temperature sensing element corresponding to each of the heat sources. The example is shown in FIG. 22. Using each temperature detected by each of the temperature sensing elements corresponding to the respective heat sources, a turn-on rate (control amount Mv) for each heater is obtained from the difference between the target temperature and the detected temperature by PID in FIG. 22 (see FIGS. 1 to 4 for this structure example).
However, in the conventional technologies as described above, when fixation is performed by allowing paper to pass through a nip between a pair of heated rotating members, an increase in temperature of the edge of the rotating members where the paper does not pass through (non-paper feeding portion) becomes larger compared to that of the portion where the paper passes when sheets of paper having a width shorter than that of the rotating members are continuously fed. More specifically, in conventional cases, the temperature of the non-paper feeding portion sometimes increases much more than necessity by turning ON two or more heat sources at the same time when the temperature is merely controlled independently such that a first heating unit (center heater) is PID controlled based on the temperature detected by a first temperature detecting unit (temperature sensing element (heating edge)) and a second heating unit (edge heater) is PID controlled based on the temperature detected by a second temperature detecting unit (temperature sensing element (heating center)), and thus there is a fear that temperature variation becomes larger. For example, when a paper size is A4, a temperature distribution in the edge of the rotating members becomes different between when the paper is transversely delivered (width of 297 millimeters) and when it is vertically delivered (width of 210 millimeters). When paper has a small width, an increase in temperature of the non-paper feeding portion becomes larger because heat is not removed from the non-paper feeding portion by the paper. In the design, the heaters are commonly set to a fixing temperature suitable for the maximum paper width (including thick paper) to secure fixation. Therefore, the temperature increase cannot be avoided. In other words, when a paper width is small, a temperature difference is generated between the paper feeding portion and the non-paper feeding portion, and the temperature of the non-paper feeding portion increases, which is not desirable in view of power consumption and safety standards. Accordingly, power saving is hindered and further a temperature increase in the apparatus is generated.